Previous internal combustion engines have employed high compression ratios to increase combustion efficiency. However, difficulties may arise when starting engines having high compression ratios. For instance, torque requirements for starting engines with high static compression ratios may surpass the available torque generated by starter motors. Low ambient temperatures causing the viscosity of the oil to be increased may further exacerbate engine starting problems. Therefore, the engine compression ratio may be varied to improve start-up operation as well as during fluctuations in engine speed and load. For example, during start-up and during high engine loads, the engine's compression ratio may be decreased. Conversely, during low engine loads, the engine compression ratio may be increased. In this way, the engine's compression ratio may be adjusted to improve overall engine performance and efficiency.
Approaches of adjusting the compression ratio in the engine may include altering geometry of the combustion chamber and/or the piston as well as modifying other engine accessories. One example approach is shown by Ma, in U.S. Pat. No. 5,101,776. Therein, a variable compression ratio engine includes a combustion chamber connected to an auxiliary chamber via a flow passage having a poppet valve. To achieve low engine compression ratios, the poppet valve may be opened to allow flow of charge gas from the combustion chamber into the auxiliary chamber. Alternatively, high compression ratios in the engine may be achieved by closing the poppet valve to isolate the auxiliary chamber from the main combustion chamber.
However, the inventors herein have recognized potential issues with Ma's system as well as other engine systems for varying compression ratio. For example, the energy contained in the charge gas bled from the main combustion chamber into the auxiliary chamber may not be adequately recovered during engine operation. Attempts have been made to extract energy from charge gas bled from the cylinder. However, previous heat recovery systems can interfere with engine starting operation. Moreover, previous heat recovery systems are not capable of independently regulating the amount of heat recovered from the combustion charge and the flowrate of the combustion charge that is bleed from the cylinder. Therefore, previous variable compression ratio systems may have drawbacks, such as engine starting problems.
In one example, the issues described above may be addressed by a method for operating an engine system. The method may include varying a compression ratio of a cylinder by selectively releasing combustion charge from the cylinder through a cylinder bleed valve of a cylinder head, the cylinder bleed valve coupled to a bleed manifold with a turbine-generator, and varying combustion charge flow through a turbine-generator bypass conduit bypassing the turbine-generator based on engine operating conditions. In this way, the engine's compression ratio can be varied by bleeding combustion charge from the cylinder during desired time periods to enable improvements in combustion operation over a wide range of conditions. From the bleed manifold combustion charge can either be routed around a turbine-generator or to the turbine-generator. The path of the charge gas may be selected to increase engine efficiency. The flow path of the combustion charge, in one example, may be chosen based on conditions such as an engine start-up condition, exhaust gas recirculation (EGR) flowrate, an energy level in an energy storage device (e.g., a battery), etc. For instance, the combustion charge may be routed around the turbine-generator during start-up operation, to promote efficient engine starting (e.g., reduce engine starting toque requirements). Consequently, the path of combustion charge bleed from the cylinder can be adjusted to achieve increased start-up efficiency.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.